Locomotive boiler draught production: In a locomotive-type multi-tubular fire-tube boiler, which device is primarily responsible for creating the draught (the pressure difference that draws air through the firebox and gases through the tubes)?

Introduction / Context:
Draught is the small but crucial pressure difference that pulls combustion air through the grate and moves hot gases through boiler heating surfaces. In stationary plants it is often produced by tall chimneys or mechanical fans, but the situation is different in locomotives.

Given Data / Assumptions:

• Boiler type: locomotive, multi-tubular, fire-tube.
• Operation: moving vehicle with limited stack height.
• Available energy source on board: exhaust steam from cylinders.

Concept / Approach:
Because a locomotive cannot carry a tall chimney to generate natural draught, it uses the kinetic energy of exhaust steam discharged through a blast pipe (steam jet) into the smokebox. The jet entrains flue gases, creating a low pressure region that pulls gases from the firebox through the tubes, thereby sustaining strong combustion even at high firing rates.

Step-by-Step Solution:
Identify draught need: continuous flow of air and gases through grate and tubes.Recognize geometric limitation: short stack height on locomotives limits natural draught.Use of exhaust: high-velocity steam jet from cylinders is directed up the stack.Effect: jet entrains gases, lowers pressure in smokebox, producing draught.

Verification / Alternative check:
Locomotive schematics show a blast nozzle and petticoat pipe; draught strength correlates with engine exhaust flow (faster running → stronger draught).

Why Other Options Are Wrong:
Centrifugal/ID fans are not standard on traditional steam locomotives; space, power supply, and maintenance make them impractical on vintage designs.

Chimney alone is too short to produce adequate draught on a locomotive.

“None of these” is incorrect because the steam jet is the established method.

Common Pitfalls:
Assuming all boilers rely on tall chimneys; forgetting that moving stock uses exhaust energy for draught.

Final Answer:
steam jet